Effective Biosensing and Phototherapy Using Upconversion Nanoparticles

Abstract

The innovative advent of nanotechnology and nanomaterials science has led to the development of platforms with a wide range of applications. Particularly, nanotechnology has revolutionized the scope of biological applications, which include the development of biosensors for the detection and monitoring of disease markers, biological metabolites, food contaminants, and environmental pollutants, and biomedical platforms for cancer therapy, bio-imaging, cell/drug delivery, and tissue engineering. The use of fluorophores as probes for biological applications is based on the generation of detectable fluorescence signals, depending on the fluorescent characteristics. However, conventional down-conversion fluorophores such as organic dyes and quantum dots (QDs) have limitations in biological applications. Therefore, the use of the suitable fluorescent probes is essential to achieving improved sensitivity and efficiency in biological applications. Upconversion nanoparticles (UCNPs) are lanthanide-doped nanomaterials that convert near-infrared (NIR) radiations into visible radiations (anti-Stokes optical properties) via a nonlinear optical process. UCNPs, which can be excited with NIR, exhibit several advantages compared to the traditional fluorescent biolabels: (1) excellent signal-to-noise ratio (SNR) and sensitivity, (2) deeper NIR light penetration into biological tissue, causing less photo damage in biological samples, (3) a sharp emission bandwidth with typical full-width-at-half-maximum (FWHM) values, (4) low toxicity, and (5) high chemical as well as physical stability. Taking advantage of these properties, the study described in this dissertation deals with the development of luminescence resonance energy transfer (LRET)-based biosensors using UCNPs as the donor for clinical diagnosis and food safety. In addition, NIR-mediated in situ photopolymerization and photodynamic therapy (PDT) were developed for biomedical applications such as bio-imaging, cell/drug delivery, construction of tissue augmenting agents, and cancer therapeutics. In Chapter 2, the development of a homogeneous LRET-based immunosensor to detect glycated hemoglobin (HbA1c) levels is described. This system was composed of UCNPs and HbA1c as the donor and acceptor of the LRET pair, respectively. The HbA1c target molecules showed absorption at 541 nm, which corresponded with the emission of the UCNPs ranging from 500 to 570 nm. In the presence of HbA1c, the specific recognition between the anti-HbA1c monoclonal antibody-functionalized UCNPs and HbA1c led to positioning of the energy donor and acceptor in close proximity by LRET, which ultimately led to quenching of the upconversion luminescence of the UCNPs. Therefore, the upconversion luminescence quenching efficiency increased gradually with increasing concentrations of the target HbA1c. Our results validated that the LRET-based immunosensor allowed for specific, sensitive, and homogeneous detection of HbA1c in blood samples. Chapter 3 describes the development of a single-probe-type LRET aptasensor for the detection of the target mycotoxin, ochratoxin A (OTA). This LRET aptasensor system was constructed with UCNPs and black hole quencher 3 (BHQ3) as the respective LRET donor and acceptor. When the colored food sample containing a target mycotoxin was added to a solution of the UCNP nanoprobe, LRET occurred between the donor and acceptor under laser irradiation of 980 nm via the specific folding of the BHQ3-labeled aptamer on the UCNP surface because of the aptamer−target complex formation. Our distinctive design of LRET-based aptasensor system allowed detection of OTA selectively in colored food samples such as wine, beer, and grape juice in 10 min without multiple bioassay steps. In Chapter 4, the development of an efficient in situ gelation method to produce bulk hydrogels via NIR-mediated photopolymerization using acrylated polyethylene glycol and diacrylated Pluronic F127 (DA-PF127)-coated UCNPs is discussed. The green emission spectrum of the UCNPs ranging from 500 to 570 nm upon 980-nm laser irradiation suitably overlapped with the absorption spectrum of eosin Y (EY), and therefore, it triggered the activation of EY to initiate the photopolymerization of vinyl group-containing precursors. Our results demonstrated that the modification of UCNPs with DA-PF127 enabled the formation of high-efficiency photopolymerized network and enhancement in colloidal stability of the UCNPs. Lastly, Chapter 5 describes the application of a nanoplatform based on conjugates of UCNPs and cancer-cell specific aptamer-photosensitizer as an effective therapeutic and diagnostic tool in PDT for target cancer cells. Overlap of the absorption of the photosensitizer (PS) and the emission of the UCNPs led to energy transfer from the UCNPs to the PS, and consequently, the upconversion luminescence of the UCNPs activated the PS towards cancer cell destruction via generation of reactive oxygen species (ROS), under NIR irradiation. Particularly, this nanoplatform enabled target-specific recognition due to an aptamer with high affinity and specificity, as well as specific destruction of the targeted cancer cells via controlled and localized ROS release from the PS by NIR-light-triggered PDT. In this dissertation, the potential use of UCNPs in LRET-based biosensors and NIR-mediated photochemical reactions has been demonstrated for many biological applications, including on-site detection of disease markers/food contaminants, and biomedical platforms for cancer therapy, bio-imaging, cell/drug delivery, and tissue engineering.